In order to alleviate impact acting on a collision object which collides with a bonnet (engine hood) exposed face up at the vehicle front portion, there has been known a technology in which a back end of a bonnet of which a leading end is supported rotatably by a vehicle body is bounced up simultaneously with collision of the collision object thereby to secure an impact-absorption stroke (refer to, for example, Patent Document 1: JP-UM-A No. 49-110432). Further, there has been known a technology in which a bonnet is composed of a double structural body, and a surface layer of the bonnet is swelled upward by fluid pressure at the collision detecting time thereby to secure an impact absorption stroke (refer to, for example, Patent Document 2: JP-A No. 10-217903). Furthermore, there has been known a technology in which a hood bag is developed so as to cover the upper surface of a bonnet when a vehicle has predicted the collision (refer to, for example, Patent Document 3: JP-A No. 8-183423).
Further, in order to alleviate impact acting on a collision object which collides with an engine hood located at the vehicle front portion, there has been known a technology in which a hood bag is developed so as to cover the upper surface of a bonnet when a vehicle predicts the collision (refer to, for example, Patent Documents 3 and 4: JP-A No. 2004-175153). In the technology in the Patent Document 4 the collision object is grasped by an adhesive layer provided for an airbag thereby to prevent the collision object from falling down on a road surface. Further, in the technology in the Patent Document 3, a gate bag developed in the arch shape is provided for an airbag, and a protection net is provided inside the gate bag to prevent the collision object from falling down on a road surface.